in considering mitosis, meiosis and fertilization can be combined in different sequences, what sequence is different than a human life cycle?
There are three cycles: haplontic, diplontic and diplobiontic. What differentiates them is the ploidy of the "main" individual: - In the haplontic cycle it is haploid (think of most protozoa), then when time for reproduction comes, the resulting diploid zygocyte immediately splits by means of meiosis into the new, haploid, individuals. Therefore, diploidy is only used for reproduction. - In the diplontic cycle, the main individual is diploid (animals and man itself), and when time for reproduction comes, gametocytes (spermatocytes and gametocytes, which are haploid cells) come into play. They are produced by meiosis, and their fusion leads to the birth of a new individual, a zygocyte which will grow by mitosis. In this case it is haploidy which is used only for reproduction. - The diplobiontic cycle is a mixture of both: there is not a preferred ploidy, in which the individual lives, and a reproduction ploidy used only for reproduction. It is most common in plants: there is a diploid form of the individual, called sporophyte, which produces spores by meiosis. The spores grow as haploid individuals, gametocytes, which grow by mitosis and, upon meeting and fusing, give birth to a diploid zygocyte which grows into a new sporophyte. Further reading: http://en.wikipedia.org/wiki/Biological_life_cycle
Oh, forgot the mitosis and meiosis part. Haplontic: Main individual (Haploid, if pluricellular grows by mitosis) -> Fecondation with another haploid individual -> Diploid zygocyte -> Meiosis -> New haploid individuals Diplontic: Main individual (Diploid, grows by mitosis) -> Produces haploid gametocytes by meiosis -> Two gametocytes fuse -> Zygocyte, which grows by meiosis into a new diploid individual. Diplobiontic: Sporophyte (Diploid, grows by mitosis) -> Produces haploid spores by meiosis -> Spores grow by mitosis into an haploid Gametophyte Gametophyte -> Two gametophitic cells fuse into a diploid zygote, which grows by mitosis into a new Sporophyte.
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